Biomass burner

ABSTRACT

A burner comprising a fuel reservoir, a primary combustion zone located at the lower end of the fuel reservoir, a secondary combustion zone located underneath the primary combustion zone, a passage providing communication between the primary and secondary combustion zones, a flue for discharging products of combustion from the secondary combustion zone, the primary combustion zone having a peripheral wall incorporating air hole means to provide combustion air to the primary combustion zone, wherein the cross-sectional area of the air hole means is substantially equal to the combined minimum cross-sectional area of the passage and the minimum cross?sectional flow area of the flue.

FIELD OF THE INVENTION

[0001] This invention relates to a burner.

[0002] The invention has been devised particularly, although notexclusively, as a burner for generating heat from combustion of lowgrade fuels including biomass material such as animal manure and“greenwaste” materials.

BACKGROUND ART

[0003] Australian patent 550050 discloses a burner for solid fuelincluding biomass material.

[0004] The invention according to patent application 550050 describes aburner having a fuel reservoir, a combustion chamber, a grate and aflue. The fuel reservoir is supported above the combustion chamber toopen into the combustion chamber at its lower end. Fuel is supported inthe grate above the combustion chamber for preheating and/or ignitionbefore entering the combustion chamber. The flue is connected into thecombustion chamber to remove combustion products. The fuel reservoir isprovided with an air inlet at a location spaced above the grate. The airinlet is typically in the form of a plurality of circumferentiallyspaced holes arranged around the walls of the fuel reservoir at itslower end.

[0005] The present invention relates to improvements to the burnerdisclosed in the above-mentioned patent.

DISCLOSURE OF THE INVENTION

[0006] The present invention provides a burner comprising a fuelreservoir, a primary combustion zone located at the lower end of thefuel reservoir, a secondary combustion zone located underneath theprimary combustion zone, a passage providing communication between theprimary and secondary combustion zones, a flue for discharging productsof combustion from the secondary combustion zone, the primary combustionzone having a peripheral wall incorporating air hole means to providecombustion air to the primary combustion zone, wherein thecross-sectional area of the air hole means is substantially equal to thecombined minimum cross-sectional area of the passage and the minimumcross-sectional flow area of the flue.

[0007] The air hole means may comprise a plurality of holes disposedcircumferentially about the primary combustion zone.

[0008] The passage providing communication between the primary andsecondary combustion zones preferably tapers inwardly in the directionfrom the primary combustion zone to the secondary combustion zone in thefashion of a funnel.

[0009] It is preferable that the primary combustion zone and thesecondary combustion zone be positioned in a manner whereby they aresituated close enough for the flames from the primary combustion zone tounite with the flames from the secondary combustion zone.

[0010] The flue is preferably provided with an inlet adjacent thesecondary combustion zone for removal of the combustion products. Thecombustion products pass from the secondary combustion zone via the flueinlet through a flue passage and exit the burner via a flue outlet.

[0011] Preferably, the minimum cross-sectional flow area of the fluecorresponds with the cross-sectional area of the flue inlet. Thecross-sectional area of the flue inlet is preferably of approximatelyequal cross-sectional area to that of the flue passage.

[0012] Preferably, a fuel grate is provided at the base of the primarycombustion zone.

[0013] Preferably, the fuel reservoir has a closable loading entrythrough which fuel can be loaded into the fuel reservoir.

[0014] Fuel loaded into the fuel reservoir rests on the fuel grate.Primary combustion occurs within the primary combustion zone above thefuel grate and is supported by combustion air supplied through the airhole means. As the waste burns, it disintegrates and falls through thegrate into the secondary combustion zone as char. The combustion of thischar, combined with the volatile gases from the fuel, produces a veryhigh and efficient reaction.

[0015] Feedstock which provides the fuel is first preheated in the fuelreservoir. This allows wet feedstock to be utilized. The feedstock isthen pyrolyzed in the primary combustion zone above the fuel grate. Theresulting pyrolysis gases are then completely oxidized in the secondarycombustion zone. Heat is internally recovered from the oxidation stepand used to preheat and dry the wet feedstock. Heat which is generatedcan also be recovered for other purposes such as generation of a hot airstream for space or processing heating.

[0016] A heat exchanger may be associated with the flue for extractionof heat energy in the products of combustion. The heat exchanger maycomprise a shell-and-tube type heat exchanger having an annular fluepassage which forms part of the flue and through which the products ofcombustion pass. The annular flue passage may be bounded by an innertube surrounding a central passage through which an air stream can passto be heated by heat transfer from the products of combustion passingalong the annular flue passage. The annular flue passage may also bebounded by an outer tube, spaced from the inner tube, located in an airreservoir containing a body of air. The air in the air reservoir may beheated by heat transfer from the products of combustion passing alongthe annular flue passage. The air reservoir may surround at least partof the fuel reservoir and upper combustion zone such that heat transferfrom the heated air in the reservoir can be used to assist drying offuel contained in the fuel reservoir. The heated air in the airreservoir may also provide the combustion air which enters the primarycombustion zone through the hole means. In this way, the combustion airis preheated.

[0017] Moisture from the drying fuel passes from the fuel reservoir andprimary combustion zone through the passage to the secondary combustionzone and exits through the flue.

[0018] An air heating chamber may be located adjacent the secondarycombustion zone, which is the hottest region within the burner. Inletair may be introduced into the air heating chamber through the centralpassage of the shell-and-tube heat exchanger. Alternatively, air may beintroduced into the air heating chamber by any other means such as afan. The heated air may be removed by means of suitable system such as afan system.

[0019] The flue may incorporate a diversion line through which at leastsome of the products of combustion exiting from the secondary combustionzone can be selectively diverted to the fuel reservoir. This assists thedrying process of wet feedstock and so accelerates the combustionprocess.

[0020] Means may be provided for pressurizing the secondary combustionzone. This will ensure that volatile pyrolysis gases remain in thesecondary combustion zone for a longer period of time so as to becompletely oxidized.

[0021] The secondary combustion zone may be pressurised by deliveringair under pressure into the secondary combustion zone.

[0022] A means may be provided for selectively blocking at least part ofthe flue to assist pressurization of the secondary combustion zone. Themeans may comprise a damper incorporated into the flue. The damper mayoperate under a control means adapted to operate the damper to achieve apredetermined pressure within the secondary combustion chamber once aprescribed operational temperature of the burner has been achieved. Thecontrol means may be in the form of a spring loaded means.

[0023] As can be seen, the burner may be used to produce heat from theburning of fuel whilst also providing a means for the thermaldestruction of the fuel which is particularly advantageous in the caseof biomass waste.

[0024] Exhaust gases from the burner contain primarily carbon dioxideand water. Accordingly, the exhaust gases are particularly suitable foruse in promoting the growth of plant life. To this end, the flue gasesmay be diverted into a greenhouse environment where plant growth can bepromoted through the process of photosynthesis which converts the carbondioxide into oxygen. Alternatively, the flue exhaust gases may firstpass to a slow release tank to allow for the controlled release ofexhaust gases to the greenhouse environment.

[0025] It is believed that this would be a particularly useful systemfor reducing greenhouse gas emissions where biomass material is used asfeedstock for the burner. Biomass material is neutral in terms of carbondioxide in that decomposition of biomass material releases carbondioxide into the atmosphere in any event and thus burning it would notadd to the net levels of carbon dioxide production. A particularadvantage of the arrangement involving passing the exhaust gases througha greenhouse environment is that the carbon dioxide so produced isconverted back into oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The invention will be better understood by reference to thefollowing description of several specific embodiments thereof as shownin the accompanying drawings in which:

[0027]FIG. 1 is a schematic view of a burner according to a firstembodiment;

[0028]FIG. 2 is a schematic view of a burner according to a secondembodiment; and

[0029]FIG. 3 is a schematic view of a burner according to a thirdembodiment.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

[0030] Referring to FIGS. 1 and 2 of the drawings, there is shown aburner 10 according to the first embodiment. The burner 10 comprises anouter housing 11 defining an upper section 13 and a lower section 15separated by an internal wall 17. The outer housing 11 is supported on abase 19.

[0031] A generally cylindrical body 21 is mounted on the outer housing11. The body 21 is primarily located in the upper section 13, althoughthe lower end thereof extends to the lower section 15 and the upper endthereof extends beyond the outer housing 11. The body 21 defines a fuelreservoir 23 and incorporates a lid 25 which can be opened to loadfeedstock into the fuel reservoir 23. The lower end section of the body21 incorporates an inwardly tapering funnel section 27 which defines apassage 28 providing communication between a primary combustion zone 29disposed at the lower end of the fuel reservoir above a fuel grate 31and a secondary combustion zone 33 located in the lower section 15 ofthe outer housing 11. The tapering section 27 terminates at a port 35which opens onto the secondary combustion zone 33.

[0032] The secondary combustion zone 33 is defined within the lowersection 15 between a partition wall 37 therein and an access door 39though which ash and other debris can be removed from the secondarycombustion zone. An air heating chamber 41 is defined in the lowersection 15 of the outer housing 11 on the opposed side of the partitionwall 37 to the secondary combustion zone 33.

[0033] The burner 10 is provided with a flue 45 having a inlet 47 whichopens onto the secondary combustion zone 33 and an outlet 49. Theproducts of combustion pass from the secondary combustion zone 33 in theflue inlet 47 through the flue passage 56 to exit via the flue outlet49.

[0034] In the second embodiment shown in FIG. 2 (which is not drawn toscale), the flue 45 is incorporated in a shell-and-tube type heatexchanger 50 designed to extract thermal energy in products ofcombustion exiting from the secondary combustion zone 33 via the flue45. The heat exchanger 50 has an inner tube 51 and an outer tube 53spaced from the inner tube 51 to define an annular flue passage 55therebetween which forms part of the flue. As will be noted, the fluepassage 56 in the first embodiment is of generally cylindrical shape,whereas the flue passage in the second embodiment is annular.

[0035] The annular flue passage 55 communicates with the flue inlet 47and the flue outlet 49 to allow the products of combustion to exit theburner. The outer tube 53 extends through the upper section 13 of outerhousing 11 as shown in the drawings. The inner tube 51 defines a centralpassage 59 through which air can be delivered to the air heating chamber41, the central passage 59 having an inlet 63 through which air entersthe passage and an outlet 65 opening into the heating chamber 41. Heatedair is extracted from the heating chamber 41 by any suitable means (notshown).

[0036] The flue 45 is provided with a flue access door 48 through whichsoot and other debris can be removed from the flue.

[0037] The upper section 13 of the outer housing 11 defines an airreservoir 52.

[0038] With the heat exchanger 50, products of combustion exiting thesecondary combustion zone 33 via the flue 45 pass in heat exchangerelationship with both the inner tube 51 to thereby heat incoming airflowing along passage 59 and the outer tube 53 to thereby heat airaccommodated in the air reservoir 52 in the upper section 13.

[0039] Heated air contained in the air reservoir 52 surrounds that partof the body 21 accommodated in the upper section 13 and so is in heatexchange relationship therewith to provide heat which assists drying ofany wet feedstock introduced into the fuel reservoir 23.

[0040] The cylindrical body 21 has a peripheral wall which incorporatesa hole means in the form of a plurality of air holes 71 providingcommunication between the air reservoir 52 and the primary combustionzone 29. The air holes 71 provide combustion air for the primarycombustion zone 29. With this arrangement, the combustion air providedto the primary combustion zone 29 from the air reservoir 52 through theair holes 71 is heated by the action of the heat exchanger 50 aspreviously explained.

[0041] The air reservoir 52 in the upper section 13 is provided with anair inlet opening 73 through which ambient air can enter the airreservoir.

[0042] The burner 10 is provided with a heated air outlet 75 for releaseof heated air.

[0043] A particular feature of the burner is that the total flow area ofthe air holes 71 providing combustion air to the primary combustion zone29 is substantially equal to the combined flow area of the port 35 inthe passage 28 between the upper and lower combustion zones and theminimum flow area of the flue 45. It should be noted that the smallestcross-sectional area of the passage 28 is located at the port 35, andthe smallest cross-sectional flow area of the flue 45 is located at theflue inlet opening 47. It has been found that this relationship isparticularly important in attaining optimal performance of the burner.

[0044] As will be seen in the first embodiment shown in FIG. 1, thesmallest cross-sectional flow area located at the flue inlet opening 47is equivalent to the cross-sectional flow area of the flue passage 56.In the embodiment shown in FIG. 2, the smallest cross-sectional flowarea of the flue 45 is located at the flue inlet opening 47 which isequivalent to the cross-sectional flow area of the annular flue passage55 although this is not apparent since FIG. 2 is not drawn to scale.

[0045] The figures show embodiments which have been configured for thegeneration of hot air. However, the burner may be adapted to use thegenerated heat in other ways. For example, to heat water the burner maybe fitted with water heating coils attached to the outside of the heatexchanger section.

[0046] Referring now to FIG. 3 of the drawings, there is shown a furtherembodiment of the burner. The embodiment shown in FIG. 3 operates in asimilar fashion to that described in the first embodiment with theaddition of several features. One such additional feature is theprovision of a diversion line 81 for diverting at least some of the fluegases leaving the secondary combustion chamber 33 along flue 45 to thefuel reservoir 23. With this arrangement, the diverted flue gases areutilized to assist the drying process of wet feedstock and so acceleratethe combustion process within the burner.

[0047] The burner according to the third embodiment incorporates afurther feature involving pressurization of the secondary combustionzone 33. The pressurization of the secondary combustion zone is achievedby delivery of air under pressure into the secondary combustion zone byany suitable means such as an electrically operated fan. It has beenfound that the rate of combustion is accelerated through suchpressurization. The pressurization ensures that all the volatilepyrolysis gases remain in the secondary combustion chamber for a longerperiod of time and are completely oxidized. Testing has demonstratedthat most of the oxygen supplied to the secondary combustion zonethrough the pressurized air is utilized in the combustion process.

[0048] A damper 85 is incorporated in the flue 45 to assistpressurization of the secondary combustion zone. The damper 85 operatesunder a control system arranged to close the damper once the burner 10has achieved a prescribed operational temperature. This can be achievedby having the damper 85 connected to a spring-loaded mechanism whichoperates to close the damper at the selected pressure.

[0049] In the embodiment shown in FIG. 3 of the drawings, the flue 45 isconnected to a greenhouse 91 by way of a delivery line 93 which deliversexhaust gases from the burner 10 into the environment within thegreenhouse. As the exhaust gases are colourless and odourless andcontain primarily carbon dioxide and water, they provide an environmentin the greenhouse which is conducive to plant growth. In the greenhouse,the carbon dioxide delivered from the burner 10 is converted into oxygenthrough the process of photosynthesis, and the converted oxygen isdischarged to atmosphere through outlet 97.

[0050] It should be appreciated that the scope of the invention is notlimited to the scope of the three embodiments described.

[0051] Throughout the specification, unless the context requiresotherwise, the word “comprise” or variations such as “comprises” or“comprising,” will be understood to imply the inclusion of a statedinteger or group of integers but not the exclusion of any other integeror group of integers.

1. A burner comprising a fuel reservoir, a primary combustion zonelocated at the lower end of the fuel reservoir, a secondary combustionzone located underneath the primary combustion zone, a passage providingcommunication between the primary and secondary combustion zones, a fluefor discharging products of combustion from the secondary combustionzone, the primary combustion zone having a peripheral wall incorporatingair hole means to provide combustion air to the primary combustion zone,wherein the cross-sectional area of the air hole means is substantiallyequal to the combined minimum cross-sectional area of the passage andthe minimum cross-sectional flow area of the flue.
 2. A burner accordingto claim 1 wherein air hole means comprises a plurality of holesdisposed circumferentially about the primary combustion zone.
 3. Aburner according to any one of the preceding claims wherein the fluecomprises a flue inlet connected to a flue outlet by means of a fluepassage.
 4. A burner according to claim 3 wherein the minimumcross-sectional flow area of the flue is at the flue inlet.
 5. A burneraccording to claim 3 wherein the minimum cross-sectional flow area ofthe flue is spaced from the flue inlet.
 6. A burner according to any oneof the preceding claims wherein the passage providing communicationbetween the primary and secondary combustion zones tapers inwardly inthe direction from the primary combustion zone to the secondarycombustion zone in the fashion of a funnel.
 7. A burner according to anyone of the preceding claims wherein the primary combustion zone and thesecondary combustion zone are positioned in a manner whereby they aresituated close enough for the flames from the primary combustion zone tounite with the flames from the secondary combustion zone.
 8. A burneraccording to any one of the preceding claims wherein a fuel grate isprovided at the base of the primary combustion zone.
 9. A burneraccording to any one of the preceding claims wherein the fuel reservoirhas a closable loading entry through which fuel can be loaded into thefuel reservoir.
 10. A burner according to any one of the precedingclaims wherein a heat exchanger is associated with the flue forextraction of heat energy in the products of combustion.
 11. A burneraccording to claim 10 wherein the heat exchanger comprises ashell-and-tube type heat recuperator having an annular flue passageforming part of the flue and through which combustion products pass. 12.A burner according to claim 11 wherein the annular flue passage isbounded by an inner tube surrounding a central passage through which anair stream can pass to be heated by heat transfer from the products ofcombustion passing along the annular flue passage.
 13. A burneraccording to claim 12 wherein the annular flue passage is bounded by anouter tube, spaced from the inner tube, located in an air reservoircontaining a body of air.
 14. A burner according to claim 13 wherein theair reservoir is heated by heat transfer from the products of combustionpassing along the annular flue passage.
 15. A burner according to claim13 or claim 14 wherein the air reservoir surrounds at least part of thefuel reservoir and upper combustion zone such that heat transfer fromthe heated air in the reservoir can be used to assist drying of fuelcontained in the fuel reservoir.
 16. A burner according to any one ofthe preceding claims wherein an air heating chamber is located adjacentthe secondary combustion zone.
 17. A burner according to claim 16wherein inlet air is introduced into the air heating chamber through thecentral passage of the shell-and-tube recuperator.
 18. A burneraccording to any one of the preceding claims wherein the flueincorporates a diversion line through which at least some of theproducts of combustion exiting from the secondary combustion zone can beselectively diverted to the fuel reservoir.
 19. A burner according toany one of the preceding claims wherein a means is provided forpressurising the secondary combustion zone.
 20. A burner according toclaim 19 wherein the means operates by way of delivering air underpressure to the secondary combustion zone.
 21. A burner according to anyone of claims 19 or 20 wherein the means operates by way of selectivelyblocking at least part of the flue.
 22. A burner according to claim 21wherein the selective blocking is effected by way of a damperincorporated into the flue.
 23. A burner according to claim 22 whereinthe damper operates under a control means to operate the damper toachieve a predetermined pressure within the secondary combustion chamberonce a prescribed operational temperature of the burner has beenachieved.
 24. A burner according to claim 22 or claim 23 wherein thedamper operates by means of a spring-loaded mechanism.
 25. A burneraccording to any one of the preceding claims wherein the burner operatesin association with a greenhouse environment.
 26. A burner according toclaim 25 wherein exhaust gases from the burner are diverted to thegreenhouse environment from the flue.
 27. A burner substantially asherein described with reference to the accompanying drawings.